Theories of health behavior are usually tested on the between-person level. Associations between variables on the between- and the within-person level, however, can differ substantially. Thus, in order to better understand intrapersonal processes in the domain of health behavior, studies applying within-person analyses are needed. This study tested the Health Action Process Approach (HAPA) on the within- and between-person level in the context of physical exercise. Design: Participants were 265 first-year students who completed nine online questionnaires every second week. Data were analyzed by focusing on intrapersonal associations applying multilevel modeling. Main Outcome Measures: Intentions for physical exercise and self-reported physical exercise served as main outcome measures. Results: Analyses mainly confirm associations specified by the HAPA at the intrapersonal level: outcome expectancies and self-efficacy, but not risk awareness, were positively associated with intentions for physical exercise. Physical exercise in turn was positively associated with intentions, self-efficacy, action control, but not with action planning. Conclusion: The HAPA could be confirmed on the within-person level. Future studies should focus on testing other theories of health behavior at the within-person level.